This invention relates to an improved foam forming process and apparatus for the manufacture of high quality fibrous webs. In one of its more specific aspects it relates to an improved method and apparatus for preparing low basis weight webs of exceptionally high uniformity, particularly tissue suitable for use as facial tissue and bathroom tissue, and in personal hygiene products.
Foam forming processes for tissue manufacture are known in the art. Among the prior art processes for producing webs by various foam forming methods are those disclosed in U.S. Pat. Nos. 3,716,449; 3,938,782; 3,871,952; and 3,837,999. These prior art patents have in common the teaching of separate foamed liquid generating systems wherein liquid containing a surface active agent is subjected to turbulence in the presence of air to create foamed liquid as carrier fluid for making up a foamed fiber furnish.
U.S. Pat. Nos. 3,876,498; 3,846,232; 4,062,721; and 3,746,613, and 4,543,156 incorporated herein by reference, disclose preferred papermaking machines useful in the process of this invention.
In a preferred embodiment of the invention, the fibrous web forming apparatus is either a papermaking machine known in the art as a crescent former or one of the twin wire type, as described in U.S. Pat. No. 4,543,156 wherein one of the forming wires acts as a turbulence generator producing the foamed liquid in which fibers are dispersed to make up the foamed fiber furnish. In these machines, foamed liquid is generated at the forming wire without the need for separate turbulence generating devices. Control of the foamed liquid as to desired air content, viscosity, specific gravity, and related characteristics is accomplished without the need for special foam generators.
U.S. Pat. Nos. 4,443,299 and 4,543,156, incorporated herein by reference, disclose processes for foam forming fibrous webs in which the foamed liquid is produced on the forming wires, stored in a silo, and totally recycled to minimize loss of surfactant from the system. In order to achieve this objective, it is necessary to dewater the wet feed pulp to a consistency in the range of 8 to 50 weight percent fiber, preferably in the range of 15 to 35 weight percent fiber, prior to formation of the desired foam and fiber furnish. Dispersion of the high consistency stock in foam in prior art has required a separate foam loop which has mixing equipment capable of relatively high shear mixing to form the desired foamed liquid and fiber furnish, also referred to herein as foamed fiber furnish. This foamed fiber furnish is very difficult to meter accurately on a dry fiber basis before it is added to the forming loop.
In the process of this invention, the foamed fiber furnish is made up from an aqueous slurry of natural or synthetic fibers or mixtures of fibers and foamed liquid carrier just prior to its introduction to the headbox. The pulp slurry supplied to the system has a consistency in the range of only about 0.5 to about 7 weight percent fibers, preferably in the range of from about 2.5 to about 4.5 weight percent. The pulp slurry is added to a foamed liquid comprising water, air and surfactant containing 55 to 80 percent air by volume forming a foamed fiber furnish having a consistency in the range of from about 0.1 to about 3 weight percent fiber by simple mixing from natural turbulence and mixing inherent in the process elements. The addition of the pulp as a low consistency slurry results in excess foamed liquid recovered from the forming wires. The excess foamed liquid is discharged from the system and may be used elsewhere or treated for recovery of surfactant therefrom by the method disclosed in a coassigned, copending patent application of Dinesh Bhat filed concurrently herewith.